Building block



FIG. 1 is a perspective view of a building block showing my new design;

FIG. 2 is a front elevational view of the design;

FIG. 3 is a rear elevational view of the design;

FIG. 4 is a left side elevational view of the design;

FIG. 5 is a right side elevational view of the design;

FIG. 6 is a top plan view of the design; and,

FIG. 7 is a bottom plan view of the design. 

The ornamental design for a building block, as shown. 